Summary: Terminally differentiated epidermal keratinocytes form the body's outer protective barrier that undergoes constant renewal as cornified squames slough off the exterior surface and are replaced by material migrating outwards from the inner living layers of the epidermis. Cornified squames are composite biomaterials with two components: a matrix of keratin filaments and the cornified cell envelope (CE). The CE is a multi-component 10nm-thick layer of insoluble protein deposited on the inner surface of the plasma membrane of the cells during terminal differentiation. In the epidermis, a 5nm-thick layer of ceramide lipids (lipid envelope) is attached to the exterior surface. The insolubility of the protein envelope is due in large part to the cross-linking of several structural proteins by transglutaminases. Intermediate filaments (IF) are ubiquitous constituents of the cytoskeletons of eukaryotic cells. They consist of five different types, of which the most numerous and complex are the type I and type II keratins widely expressed in epithelia. We are interested also in IFs of other cell types and in the interactions between keratin IF and the CE. The long-term goal of this project was to elucidate the production, assembly, structure and biomechanical properties of these cells and their components in the context of both normal and diseased skin. This project was in abeyance for several years but in FY16 we reactivated and completed it, publishing a study that demonstrates how, when loricrin, the major protein component of normal CEs, is knocked out, the expression of Lce1 proteins is upregulated and these proteins are incorporated into the CE where they substitute for loricrin.